Display device having an electrode partially covering a picture element

ABSTRACT

An optical switch, e.g., a display cell based on layer displacement or layer break up having at least two different states, in which one fluid, e.g. oil, in a first state adjoins at least a first support plate, and in the second state, another fluid at least partly adjoins the first support plate. Part of an otherwise homogeneous electrode is removed to realize oil motion control.

The invention relates to an optical switch having at least one firstfluid and a second fluid immiscible with each other within a spacebetween a first transparent support plate and a second support plate,the second fluid being electroconductive or polar. In particular theinvention relates to a display device comprising picture elements havingat least one first fluid and a second fluid immiscible with each otherwithin a space between a first transparent support plate and a secondsupport plate, the second fluid being electroconductive or polar.

Optical switches may be used in shutter applications, diaphragms, butalso in switchable color filters in e.g. display applications.

Display devices like TFT-LCDs are used in laptop computers and inorganizers, but also find an increasingly wider application in GSMtelephones. Instead of LCDs, for example, (polymer) LED display devicesare also being used.

Apart from these display effects which are well established by now otherdisplay techniques are evolving like electrophoretic displays, which aresuitable for paper white applications.

The invention is based on a principle called electro-wetting. Theinvention provides new ways of using this principle.

If for instance a (first) fluid is a (colored) oil and the second (theother) fluid is water (due to interfacial tensions) a two layer systemis provided which comprises a water layer and an oil layer. However, ifa voltage is applied between the water and an electrode on the firstsupport plate the oil layer moves aside or breaks up due toelectrostatic forces. Since parts of the water now penetrate the oillayer the picture element becomes partly transparent.

If homogeneous (Indium Tin Oxide) electrodes are used to address thepicture elements, since this is the simplest way, the electric field isapplied across the entire picture element Hence, in principle, there isno preferred direction for the (first) fluid (the oil) to move to.However, in practice there will always be a (small) inhomogeneity insidethe picture element (insulator or oil thickness variation, slightirregularity in a pixel wall, etcetera) that will determine which waythe (first) fluid (the oil) will move. As a result always the samemotion occurs upon voltage application. However, the motion will varyfrom pixel to pixel. For several reasons, including grey-scalehomogeneity and reducing the chance of oil mixing between adjacentpixels, the oil motion should be better controlled.

To this end a display device according to the invention comprises withineach picture element a first electrode only partly covering the totalarea of the picture element Preferably the first electrode at leastleaves clear a part of the picture element along an edge of the pictureelement.

When a voltage is applied, there will be an electric field at the placeswhere the electrode is present Effectively this region will become morewettable for the second fluid (more hydrophilic in an oil-water system).On the other hand, at places where no electrode is present there will beno electric field, and thus this region will be less wettable for thesecond fluid (more hydrophobic). As a result, the oil will tend to movetoward the least wettable region.

In a particular embodiment a display device according to the inventioncomprises at least one further electrode at the remaining area of thepicture element. If driving means are present to apply voltages to thefirst and further electrodes the oil motion can be enhanced.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1 is a diagrammatic cross-section of a part of a display deviceaccording to the invention,

FIG. 2 is another diagrammatic cross-section of a part of a displaydevice according to the invention, while

FIGS. 3 and 4 are plan views of a part of a display device according tothe invention and

FIG. 5 shows driving voltages

The Figures are diagrammatic and not drawn to scale. Correspondingelements are generally denoted by the same reference numerals.

FIG. 1 shows a diagrammatic cross-section of a part of a display device1 according to the invention. Between two transparent substrates orsupport plates 3, 4 a first fluid 5 and a second fluid 6 are provided,which are immiscible with each other. The first fluid 5 is for instancean alkane Like tetradecane or hexadecane or as in this example a(silicone) oil. The second fluid 6 is electroconductive or polar, forinstance water or a salt solution (e.g. a solution of KCl in a mixtureof water and ethyl alcohol).

In a first state, when no external voltage is applied (FIG. 1 a) thefluids 5, 6 adjoin the first and second transparent support plates 3, 4of e.g. glass or plastic. On the first support plate 3 a (transparent)electrode 7, for example indium (tin) oxide is provided and anintermediate less wettable (hydrophobic) layer 8, in this example anamorphous fluoropolymer (AF1600).

When a voltage is applied (voltage source 9) via interconnections 20, 21the layer 5 moves aside or breaks up into small droplets (FIG. 1 b,fragmented film). This occurs when the electrostatic energy gain islarger than the surface energy loss due to the creation of curvedsurfaces. As a very important aspect it was found that reversibleswitching between a continuous film 5 covering the support plate 3 and afilm adjoining the wall 2 is achieved by means of the electricalswitching means (voltage source 9).

FIG. 2 shows an embodiment of a display device according to theinvention, in which part of the electrode 7 has been left away (see alsoFIG. 3). When a voltage is applied, there will be an electric field atthe places where the electrode (ITO) is present. In this region thecoating will become more wettable for the second fluid. On the otherhand, at the lower left hand corner (FIG. 3) where there is no electrode(ITO) present there will be no electric field, and thus the coating willremain less wettable. As a result, the oil will tend to move toward theleast wettable region: the lower left hand corner. As a result, alwaysthe same motion towards especially designed corners of display cellsoccurs upon voltage application.

Voltages are applied by means of a driving unit 22. The size of the areaof ITO that is removed should be chosen properly. When a large area ischosen, the remnant oil area will consequently be rather large, therebyreducing the brightness of the display. On the other hand, when thechosen area becomes too small, the oil motion will no longer have apreference to move into this corner. Preferably at least a part alongthe edge of the picture element comprising 5% of the total area of thepicture element is kept clear, while at most a part along the edge ofthe picture element comprising 50% (preferably 10%) of the total area ofthe picture element is kept clear.

In a matrix driven display device a matrix of switching elements e. g.thin film transistors (TFT) may be chosen for applying the drivingsignals. The thin film transistors preferably are located at the areaswhere no electrodes 7 are present. If necessary this area (and alsointer-pixel areas) may be covered by a black matrix to enhance contrast.

FIG. 4 shows another embodiment in which a corner of the picture elementhas been provide with a further electrode 7′. Voltage pulses (see FIG.5) are provided to electrodes 7, 7′ to accelerate the retraction of theoil film. First, a voltage pulse 15 is applied to electrode 7 to removethe oil from most of the active area Electrode 7′ is grounded and as aresult, the oil will move into the corresponding corner of the pictureelement. After the voltage on electrode 7 has been removed, a shortvoltage pulse 16 is applied to electrode 7′. As a result, the oil isremoved from electrode 7′ and redistributed over the rest of the pictureelement. When the voltage on electrode 7′ is removed, the oil will alsospread over this part of the picture element.

The electrode configuration chosen here is merely an example. Otherelectrode configurations can be chosen, such as a circular geometry.Such a circular geometry is used in e.g. shutter applications anddiaphragms.

The electrode on which the oil is collected should be as small aspossible compared to the total size, while still being sufficientlylarge to determine the direction of motion.

The invention resides in each and every novel characteristic feature andeach and every combination of characteristic features. Referencenumerals in the claims do not limit their protective scope. Use of theverb “to comprise” and its conjugations does not exclude the presence ofelements other than those stated in the claims. Use of the article “a”or “an” preceding an element does not exclude the presence of aplurality of such elements.

1. An optical switch for a picture element, the optical switchcomprising: a first transparent support plate; a second support plate; afirst fluid; a second fluid immiscible with the first fluid within aspace between the first transparent support plate and the second supportplate, the second fluid being electroconductive or polar; a firstelectrode on at least one of the support plates, the first electrodeonly partly covering a total area of the picture element leaving aremaining part of the optical switch not covered by the first electrode;and a second electrode covering at least a portion of the remainingpart; and a pulse generator configured to provide to the first electrodea first pulse having a first pulse width while the second electrode isgrounded and, after removal of the first pulse, to provide to the secondelectrode a second pulse having a second pulse width, wherein the secondpulse width is smaller than the first pulse width; and wherein the firstelectrode is substantially along an entire first edge and an entiresecond edge of the optical switch, and the second electrode is along athird edge of the optical switch.
 2. The optical switch according toclaim 1, wherein the remaining part is one of the corners of asubstantially rectangular optical switch.
 3. The optical switchaccording to claim 1, wherein the remaining part comprises 5% of a totalarea of the optical switch.
 4. The optical switch according to claim 1,wherein the remaining part comprises 50% of a total area of the opticalswitch.
 5. The optical switch according to claim 1, wherein theremaining part comprises 10% of a total area of the optical switch. 6.The optical switch according to claim 1, further comprising at least onefurther electrode at the remaining part of the optical switch.
 7. Theoptical switch according to claim 6, further comprising driving means toapply voltages to the first electrode and the further electrode.
 8. Adisplay device comprising picture elements, the picture element havingthe optical switch according to claim
 1. 9. The display device accordingto claim 8, further comprising switching devices at the remaining partleft clear by the first electrode.
 10. The display device according toclaim 9, wherein the remaining part left clear by the first electrode iscovered by a black mask.
 11. A display comprising a plurality of pictureelements, wherein each picture element includes: two fluids which areimmiscible with each other located between support plates; a firstelectrode on at least one of the support plates, the first electrodeonly partly covering a total area of the picture element, leaving aremaining area not covered by the first electrode, wherein the pictureelements have remaining areas at substantially same positions so that afirst fluid of the two fluids moves towards the same positions of thepicture elements upon application of a voltage to the first electrode,wherein the first electrode is substantially along an entire first edgeand an entire second edge of the picture element, and the remaining areais along a third edge of the picture element; a second electrodecovering at least a portion of the remaining area; and a pulse generatorconfigured to provide to the first electrode a first pulse having afirst pulse width while the second electrode is grounded and, afterremoval of the first pulse, to provide to the second electrode a secondpulse having a second pulse width, wherein the second pulse width issmaller than the first pulse width.
 12. The display of claim 11, whereinthe same positions includes the third edge of each of the pictureelements.
 13. The display of claim 11, wherein the same positionsincludes at least one corner of each of the picture elements.
 14. Thedisplay of claim 11, wherein the each picture element further includes asecond electrode located at the remaining area, so that application of avoltage to the second electrode moves the first fluid away from theremaining area.
 15. The display of claim 14, wherein the first fluid isoil which is spread over remaining area upon removal of the voltage tothe second electrode.